The tropics are harsh on fly lines.
Most lines are made with limp, braided cores and softer coatings for excellent handling in the cold to modest climates that many folks face much of the time. In the tropics, these "standard" lines can quickly become a massive frustration to use due to their surfaces becoming gummy and wilted from the heat. Rather than flowing smoothly, the line sticks to the rod guides and casting generally just becomes a chore that's not enjoyable—I speak from experience here!
Tropical lines are manufactured with monofilament cores for stiffness along with harder coatings to keep them fishable and composed in hot, humid conditions. These lines are indeed highly-specialized, as the ones I've used get quite stiff and crinkly in the more temperate conditions where standard lines shine. But, if fishing in extreme heat, tropical lines are indeed a necessity if you want to fish with efficiency.
You don't have to fish in the tropics to appreciate tropical-style fly lines.
Living in South Florida, I certainly have an arsenal of them that I use heavily from about May thru October. However, don't let the name "tropical" fool you into thinking these lines are only useful the closer to the equator you are. I've also used my tropical fly lines during trips to Arizona and Northern California in summer when air temps were over 100F and water temps well over 80F. It doesn't matter where you are—if the air and water temps are extreme it can affect fly line performance, especially through the middle of the day when the sun is really cooking.
Panfish, bass, and carp are often found in areas more susceptible to high air and water temps. Because of this fact, the folks targeting these species could seemingly benefit greatly from tropical lines during the hottest days of the year. But what line to get? Tropical lines are typically marketed towards saltwater applications, but there are also some made for anglers targeting jungle species like peacock bass. Again, just because there's a particular name and picture on a box doesn't mean that's ALL the line is good for.
Tropical lines start as low as WF5F and range up to weights suitable for big tarpon and billfish. The lightest line weights and less-aggressive designs will typically be found in the lines designed for bonefish or multi-purpose saltwater duties. These lines can be great for a lot of general panfish, carp, and bass fishing since their tapers are relatively versatile.
If you want a more aggressive line for throwing a bit more substantial flies, something with a shorter, more powerful head would be a fine place to start. Lines designed for redfish would be a good example here. These are often made with shorter, stouter heads that load quickly and cast with increased authority—great for casting larger/heavier flies, especially at close range. A similar line I've caught many bass with is RIO's Bonefish Quickshooter line. It's unique in that it has a shorter, more compact head compared to most bonefish lines, but it's also much heavier than AFFTA standards which does well when target casting with chunkier bugs.
Anglers needing to fish deep shouldn't feel left out, either. There's tropical varieties of these lines available as well. The major line manufacturers offer tropical styles of intermediate, sink-tip, and full-sinking lines that are made stiffer and firmer just like their floating counterparts.
If you've ever had your fly line become a sticky, gummy mess in high heat, you'll know how maddening that can be. A tropical fly line solves that issue instantly. Even if you'd only use such a line a handful of times per year where you fish, it's worth its weight in gold when you really need it. These lines pull double-duty very well not just for the sub-tropics or tropics, but also for those strong heat waves you might have back home that can make your other lines a pain to use. So, whether you already own tropical lines or are thinking of making a purchase, consider trying one anytime you face extreme air and water temps—no matter what the location or species!
Some great tropical lines can be found at Trouts Fly Fishing and Fishwest